Super absorbent polymer (SAP) is a synthetic polymer material having a function of absorbing water about 5 hundred times to about 1 thousand times of the weight of itself, and it has been differently named as super absorbency material (SAM), absorbent gel material (AGM), and so on by developing enterprises. The SAP disclosed above was started to be commercialized for sanitary items and is now being used widely to a water combination soil for horticulture, a water-stop material for civil engineering and construction, a nursery sheet, a freshness preservative in a food distribution field, a poultice material, and the like in addition to the sanitary fittings like a paper diaper for a child.
An inverse suspension polymerization method or an aqueous polymerization method is known as a method of preparing super absorbent polymer. For example, the inverse suspension polymerization is disclosed in Japanese Patent Publication Nos. Sho56-161408, Sho57-158209, Sho57-198714, and so on. As the aqueous polymerization method, a thermal polymerization method that polymerizes a polymer gel while fracturing and cooling the same in a kneader equipped with a spindle, and a photo-polymerization method that exposes a high-concentrated aqueous solution on a belt to UV rays and the like so as to carry out the polymerization and the dry at the same time are known.
Japanese Patent Publication No. 2004-250689 discloses a preparation method of an absorbent polymer that carries out the polymerization by intermittently irradiating the aqueous solution including a photoinitiator and a water-soluble ethylenic unsaturated monomer with light. And, Korean Patent No. 0330127 discloses a preparation method of an absorbent polymer that carries out the polymerization by irradiating a water-soluble ethylenic unsaturated monomer including a cross-linking agent with UV rays in the presence of a radical photoinitiator having a benzoyl group and peroxide.
As a prior method of using the UV polymerization, the method of feeding a monomer composition including a monomer and an initiator to a conveyor belt reactor 1; preparing a base resin 2, a hydrogel polymer in sheet form, by carrying out the reaction within several seconds by using UV rays; and preparing a powdery SAP by passing through several post-treatment processes has been used. Said post-treatment processes may include the processes of gel-sizing, sizing (cutting process), drying, pulverizing, surface treatment, and so on.
However, said method must include a recycling process repeating the processes of gel-sizing, drying, and sizing several times for obtaining the hydrogel polymer in fine particles. But there is a problem that the property of the SAP deteriorates because of the generation of fine particles and the additional process is required for the recycle, when the recycling process is carried out several times as disclosed above in order to control the diameter. Namely, there is a problem that the prior method may reduce the properties of the SAP and the overall process is inconvenient and complicated because the SAP having a unnecessary particle size distribution is formed in the processes of classifying-pulverizing and drying the gel polymer in order to prepare the SAP having a certain degree of particle size distribution adequate to a product.
Moreover, since the method is carried out by a 2-dimensional curing system after the monomer composition of the monomer and the initiator is fed in a liquid-phase, it has a problem of limitation on the thickness of the monomer composition according to the transmittance of light in the UV polymerization.